1. Field of the Invention
The present invention relates to a process for preparing 1,1-dichlorofluoroethane (HCFC-141b) and 1-chloro-1,1-difluoroethane (HCFC-142b) from 1,1,1-trichloroethane and hydrogen fluoride. More particularly, the present invention relates to a new process for preparing HCFC-141b and HCFC-142b, which is capable of increasing the reaction conversion and selectivity of the reaction products, HCFC-141b and HCFC-142b. In the process according to the invention, a known, non-catalytic reaction of 1,1,1-trichloroethane with hydrogen fluoride is employed.
2. Description of the Prior Art
HCFC-141b and HCFC-142b have remarkably lower ozone depletion potentials than chlorofluorocarbons which are used as foaming agents, refrigerants, and the like. HCFC-141b has outstanding physical properties for a polyurethane foaming agent and a cleaning solvent for electronic circuits. HCFC-142b is used as a refrigerating medium and a propellant for aerosols. Thus, recently, demands of these materials tend to be gradually increasing.
In general, it is known that HCFC-141b and HCFC-142b are simultaneously obtained when reacting 1,1,1-trichloroethane with hydrogen fluoride in the presence or in the absence of a catalyst.
Methods of preparing HCFC-141b and HCFC-142b in the presence of a catalyst are disclosed in Japanese Patent Publication 59-46211 wherein an antimony halide catalyst is employed, and in U.S. Pat. No. 4,258,225 wherein a thallium halide is used. In these methods, chlorine must be periodically supplied into the reaction system because of the gradual degradation of the catalyst; this may cause undesired side-reactions formed.
In U.S. Pat. No. 4,849,555, use of perfluoroalkanesulfonic acid as a catalyst is disclosed. This method is not practical because it requires an expensive catalyst and a high reaction pressure of above 30 atm.
Korean Laid-Open Patent Publication 91-7847 discloses a method of manufacturing only HCFC-142b using an antimony halide catalyst.
In order to avoid the formation of high molecular weight by-products (in most cases, products having at least 4 carbon atoms) and deactivation of the catalysts, non-catalytic processes have been suggested. For example, U.S. Pat. No. 3,833,676 proposes a method of using an excess of hydrogen fluoride in the absence of a catalyst; however, this method suffers from the problems involved in the recovery and recycling of the excess hydrogen fluoride.
In Japanese Laid-Open Patent Publication 2-152935, the amount of hydrogen fluoride used is significantly reduced, but the reaction time is extended considerably due to the low reaction rate.
Further, Japanese Laid-Open Patent Publication 58-217403 discloses in detail the way to recover hydrogen fluoride during the production of HCFC-141b and HCFC-142b from 1,1,1-trichloroethane, and to solve the problem of the formation of tars.
However, the prior art methods mentioned above failed to meet the requirements for increasing reactivity and minimizing the formation of the tars.
In general, synthesis of HCFC-141b and HCFC-142b from 1,1,1-trichloroethane and hydrogen fluoride is carried out stepwise in accordance with the following reactions: